To ease pollution, it is advantageous to use hydrogen as the fuel for various mobile and stationary engines, and fuel cells. The source of hydrogen can be from the electrolysis of water, from the transformation of hydrocarbon fuels such as gasoline or natural gas, or the like. The concentration of hydrogen in a fuel gas or the like is an important parameter that is preferably carefully, rapidly and accurately monitored. For example, in hydrogen fuel cells, the concentration of hydrogen is continuously monitored for process control.
The automotive industry has used various gas sensors in automotive vehicles for many years. For example, electrochemical sensors based on polarographic principles have been developed for determining the concentration of oxygen or unburned components in exhaust gases produced by an internal combustion engine or a motor vehicle. These types of oxygen sensors typically include a pump cell and a Nernst cell built, for example, from solid oxide electrolyte materials such as doped zirconia, and linked together through an external electrical circuit. The Nernst cell includes an air reference electrode (or a biased reference electrode) and a sensing electrode with a solid electrolyte therebetween. The pump cell includes a first and second electrode with a solid electrolyte therebetween and a gas chamber with an aperture. The first electrode of the pump cell and the sensing electrode of the Nernst cell are exposed to the gas chamber that receives a representative flow of test gas, such as engine exhaust gas. A controlled electrical potential is applied to the pump cell to pump oxygen into and out of the gas chamber to maintain the electromotive force of the Nernst cell as sensed at the air reference electrode thereof at a desired potential.
To provide for sensing of the oxygen concentration in the test gas, such as by sensing oxygen flux in the gas chamber, the sensor must be maintained in a current limiting range of operation by maintaining the Nernst potential applied to the sensor within a predetermined voltage range. The current limiting range of operation is characterized by a sensor output current that is insensitive to variations in the potential applied to the pump cell. In such a range of operation, the aperture limits gas flux into or out of the gas chamber and sensor output current indicates the maximum flow that can be supported by the concentration in the test gas. If the potential is above the predetermined Nernst voltage range, additional oxygen may be stripped from gas species such as water (H2O) and carbon dioxide (CO2), skewing the relationship between the gas concentration and sensor output current. If the potential is below the predetermined Nernst voltage range, an excess of oxygen is available and sensor output current does not indicate oxygen concentration but rather is a nonlinear function of the gas concentration.
Current sensors such as the oxygen sensors described above are inadequate for determining hydrogen concentration over a wide range of concentrations. For example, zirconia is a solid-state electrolyte material frequently used in the manufacture of oxygen sensors. In these applications, the electrolyte material conducts oxide ions not protons. In contrast, for hydrogen sensing the electrolyte material is a proton-conducting electrolyte, especially in oxygen-deficient atmospheres. However, many of the electrolyte materials used in oxygen gas sensors exhibit poor stability or do not exhibit sufficient conductivity. For example, barium ceria, barium zirconia, strontium ceria, and strontium zirconia are not stable when fuel gas contains water vapor or carbon dioxide. As a result, sensors employing these materials have limited applications, because either the electrolyte materials are instable and have tendency to decompose in the fuel gas environment, or the conductivity of the materials is too low to be practical for sensing applications. Moreover, when the hydrogen concentration approaches 100%, the pump current approaches an infinite number because the gas cell surrounded by the diffusion-limiting barrier becomes a vacuum and the hydrogen has no diffusion limitation into the cell. Thus, it is desirable to have a hydrogen sensing device that is stable, exhibits high conductivity to permit operation at temperatures as low as about 450 to 500xc2x0 C. and is sensitive to hydrogen concentrations over a wide range (e.g., 0% to 100%).
A method of measuring a hydrogen concentration in a gas comprises exposing a hydrogen sensor to the gas. The hydrogen sensor includes a pump cell, a measuring cell, and an insulating layer disposed between the pump cell and the measuring cell. The pump cell comprises a first pump electrode exposed to the gas, a second pump electrode in operable communication with a diffusion-limiting barrier, and a first conducting electrolyte disposed between the first and second pump electrodes. The measuring cell comprises a sensing electrode in operable communication with the diffusion-limiting barrier, a reference electrode in fluid communication with a reference gas source and a second conducting electrolyte disposed between the sensing and the reference electrodes. The diffusion-limiting barrier has a pore size sufficient to produce a Knudsen diffusion mechanism at hydrogen concentrations greater than about 40%. The process further includes applying a voltage to the first and the second pump electrodes to form a pump current; diffusing hydrogen molecules across the diffusion-limiting barrier; generating an electromotive force signal between the sensing electrode and the reference electrode; and adjusting the pump current to maintain the electromotive force signal at a predetermined value, wherein the hydrogen concentration is proportional to the pumping current.
In another embodiment, a hydrogen gas sensor comprises a pump cell and a measuring cell. The pump cell comprises a first electrode and a second electrode, and a first conducting electrolyte layer interposed between the first and the second electrode, wherein the first electrode is in fluid communication with a testing gas. The measuring cell, in operable communication with the pump cell, comprises a third and a fourth electrode, and a second conducting electrolyte layer interposed between the third and fourth electrodes, wherein the fourth electrode is in fluid communication with a reference gas. A diffusion-limiting barrier is disposed in fluid communication with the pump cell second electrode and the measuring cell third electrode, wherein the diffusion-limiting barrier has a pore size sufficient to produce a Knudsen diffusion mechanism at hydrogen concentrations greater than about 40%. An insulating layer is interposed between the pump cell and the measuring cell, wherein the insulating layer comprises a via coaxial and in fluid communication with the pump cell second electrode, the diffusion limiting barrier and the measuring cell third electrode.
In another embodiment, a hydrogen sensor comprises means for applying an electrical potential to a pump cell for migrating hydrogen molecules across a diffusion limiting barrier and into a measuring cell, wherein the diffusion limiting barrier comprises a pore size sufficient to produce a Knudsen diffusion mechanism at hydrogen ion concentrations greater than about 40%; and means for maintaining an electromotive force in the measuring cell, wherein the electrical potential is proportional to a concentration difference of the hydrogen molecules between a test gas and a reference gas.
The above described and other features are exemplified by the following figures and detailed description.